One such detector in which the secondary vacuum pump is a molecular diffusion pump is described in the "Porta-test" operating instructions dated Mar. 14, 1979 and accompanying the Porta-test helium leak detector made by Varian under the reference 0925K0777-301. When such a detector is operating in counterflow mode, the diffusion pump must operate under reduced temperature conditions to enable a counterflow of helium to pass, thus giving rise to poor stability in the helium signal. In that document, the detector is constituted by two boxes, one of which constitutes its main electronic module.
An object of the present invention is to provide a detector of the above type having a helium signal which is more stable, which is extremely simple and reliable in operation, preventing any possibility of misoperation and thus suitable for being used by non-qualified personnel, and also making it possible to save time between two successive leak tests.